Alfalfa is a perennial forage crop grown for its high nutritional value, but it is highly susceptible to competition from weeds. Weed infestations reduce forage yield, lower hay quality, and shorten the productive life of the field. Since weeds compete directly with alfalfa for water, sunlight, and nutrients, chemical control is often necessary for profitable commercial production. Effective weed management requires understanding the specific weed threats and the appropriate timing for herbicide application.
Identifying Target Weeds and Application Timing
The first step in chemical weed control is accurately identifying the weed species and classifying them as grass or broadleaf, and as annual or perennial. This classification is foundational because the herbicide must be selective enough to kill the weed without harming the alfalfa, which is a broadleaf plant. Uncontrolled weeds weaken young alfalfa plants, retard growth, and lead to significant stand loss, especially during establishment.
The alfalfa stand’s growth stage dictates the narrow window for safe herbicide application, as chemicals safe for established stands often injure new seedlings. The main application windows are during the dormant season, between cuttings in established stands, and during new stand establishment. Applying herbicides to dormant alfalfa in late fall or early spring, before significant new growth, allows for the use of more potent residual herbicides.
The post-harvest window is the only opportunity for application between cuttings in established stands, but timing is restrictive; alfalfa regrowth must be minimal so the spray reaches the weeds and soil. New seedlings have the most limited chemical options, and treatments are applied when the alfalfa reaches the second or third trifoliate leaf stage. Successful chemical control hinges on matching the herbicide’s activity to the weed type and the alfalfa’s current growth stage.
Herbicides for Grass Weed Control
Controlling grassy weeds in alfalfa uses highly selective post-emergence herbicides that capitalize on the difference between grass and broadleaf alfalfa. Clethodim and sethoxydim are common options used to control annual and perennial grasses in established alfalfa. These compounds are Acetyl-CoA carboxylase (ACCase) inhibitors, which disrupt the production of lipids needed for cell membrane formation in grasses.
Because alfalfa is a broadleaf plant, it does not possess the same ACCase enzyme structure, making it naturally tolerant to these herbicides. These grass-specific products are most effective when applied to young, actively growing grasses, generally before they develop tillers. For perennial grasses, a higher application rate may be necessary to control the rhizomes and underground structures.
Some pre-emergence chemicals are also available for grass control, such as pendimethalin, which can be applied to established alfalfa in the fall after the last cutting or during winter dormancy. This provides a layer of residual control against germinating annual grasses like foxtails and crabgrass. In the case of Roundup Ready alfalfa varieties, the non-selective herbicide glyphosate can be used over the top to control most grass species at any growth stage.
Herbicides for Broadleaf Weed Control
Controlling broadleaf weeds is challenging because alfalfa is a broadleaf legume, requiring careful selection of selective chemistry. For pre-emergence control, residual herbicides like metribuzin or hexazinone are applied to established, dormant alfalfa stands. These products control winter annual broadleaves such as chickweed and henbit, and application must occur before spring regrowth begins to prevent crop injury.
For emerged broadleaf weeds in established stands, the growth regulator herbicide 2,4-DB is a common post-emergence choice. This chemical mimics natural plant hormones, causing lethal growth in susceptible broadleaf weeds, but alfalfa metabolizes the compound into an inactive form. Application of 2,4-DB is limited to when annual broadleaf weeds are small and the alfalfa is actively growing, usually after reaching the second or third trifoliate leaf stage.
In both seedling and established alfalfa, the active ingredients imazethapyr and imazamox offer broader post-emergence control of many broadleaf species and some grasses. These chemicals provide some residual activity, but they are most effective when applied to small, actively growing weeds. The use of these particular herbicides can sometimes cause temporary growth reduction in the alfalfa, but the benefit of weed control usually outweighs the minimal crop setback.
Safe and Effective Herbicide Application
Successful weed control depends on selecting the correct chemical and ensuring safe, effective application according to the product label. The application rate must be precise; too little product results in poor control, while too much can cause crop injury or illegal residue. Calibrating the sprayer to deliver the correct volume per acre is a necessary step before application begins.
Environmental conditions, particularly wind speed and temperature, must be monitored carefully to prevent herbicide drift onto adjacent crops or sensitive areas. Temperature inversions, which often occur in the early morning or late evening, can trap small spray droplets near the ground and carry them long distances. Many post-emergence herbicides also require the addition of a crop oil concentrate or surfactant to maximize absorption by the weed leaves.
A mandatory consideration is the Pre-Harvest Interval (PHI), the minimum number of days that must pass between herbicide application and cutting or grazing. PHIs range from a few days to over a month, and observing this waiting period is a legal requirement to ensure forage residue levels are safe for livestock consumption. The herbicide label is the legal document governing product use, and all instructions regarding application timing, rate, and PHI must be followed exactly.